In the configuration where the primary side of a wound-rotor induction machine is connected to the electric power system, and the secondary side is excited by an AC current with a slip frequency, such as a static Scherbius system or a super-synchronous Scherbius system, if the primary side of the wound-rotor induction machine becomes unbalanced, an inverse-phase current flows in, and a current with a frequency of 2f1±sf1 (where f1 is a frequency of the electric power system, and s is a slip) flows into the secondary side. If a DC current component flows into the primary side, a current of fr (rotation frequency of a rotor) flows into the secondary side.
However, it is difficult for a frequency converter which excites the secondary side of the wound-rotor induction machine by an AC current with the slip frequency to cause a current with a frequency of 2f1±sf1 or fr to flow into the secondary side. In this case, a power semiconductor element (e.g., GTO or IGBT) within the frequency converter cannot be turned on, and the secondary side of the wound-rotor induction machine is opened, thereby producing an abnormally high voltage. If a high voltage occurs in the secondary side of the wound-rotor induction machine, the dielectric breakdown may occur in the secondary winding and the frequency converter.
To solve the above problem, there is a method for protecting a secondary winding or a frequency converter from an over-voltage by directly short-circuiting the secondary winding or short-circuiting the secondary winding through a resistance to suppress the voltage to be nearly zero when an over-voltage occurs at the secondary side of the wound-rotor induction machine.
FIG. 4 shows an example of a circuit structure including a secondary over-voltage prevention device of the conventional wound-rotor induction machines.
As shown in FIG. 4, a primary winding terminal of a wound-rotor induction machine 1 is connected to an electric power system 3 through a main transformer 2 and a power transmission line 4. A voltage of the primary winding terminal of a wound-rotor induction machine 1 is converted to a voltage equivalent to that of the electric power system 3 at the main transformer 2, and then supplied to the electric power system 3 through the power transmission line 4.
The secondary winding terminal of the wound-rotor induction machine 1 is connected, through a first short-circuit device 12, to a frequency converter 7 comprising, for example, a self-excited converter 5/inverter 6 connected to the main transformer 2.
At the frequency converter 7, a three-phase AC voltage is converted to a DC voltage by the converter 5, and the DC voltage is stored at the DC link capacitor 8. In addition, the DC voltage is then converted to a three-phase AC voltage corresponding to a slip frequency of the wound-rotor induction machine 1 by the inverter 6. The frequency converter 7 excites the secondary side of the wound-rotor induction machine 1 by the three-phase AC voltage. The frequency converter 7 comprises a chopper 11 including a resistor 9 and a power semiconductor element 10 (e.g., GTO or IGBT) to protect an element forming the converter 5 or the inverter 6 from an over-voltage due to an increase of DC link voltage.
A first short-circuit device 12 is provided between the frequency converter 7 and the secondary winding terminal of the wound-rotor induction machine 1. The first short-circuit device 12 has a function of short-circuiting between phases of three-phase AC currents when an over-voltage is produced at the secondary side of the wound-rotor induction machine 1.
Next, the operation of the secondary over-voltage prevention device of the wound-rotor induction machine 1 having the aforementioned configuration will be explained with reference to the timing chart of FIG. 5.
It is assumed that a failure occurs in the electric power system 3 or the power transmission line 4 at the time t1, and the primary side of the wound-rotor induction machine 1 becomes unbalanced, and an AC current of 2f1±sf1 is produced at the secondary side due to the inverse phase component. In this case, the frequency converter 7 cannot tolerate the AC current, and the secondary winding of the wound-rotor induction machine 1 becomes momentarily opened, thereby producing an over-voltage. The over-voltage is rectified by a diode of the inverter 6 of the frequency converter 7, and the DC link capacitor 8 is recharged. Accordingly, the capacitor voltage increases.
If an over-voltage above a threshold is produced at the secondary winding of the wound-rotor induction machine 1 or the DC link capacitor 8, the chopper 11 and the short-circuit device 12 are activated at the time t2. In an actual case, the chopper 11 is activated first, and if the over-voltage is not eliminated after a predetermined time has elapsed, the first short-circuit device 12 is activated; however, the illustration is simplified so that the overall operation may be easily understood.
By the above operation, the first short-circuit device 12 performs three-phase short-circuiting to the secondary winding of the wound-rotor induction machine 1 and the output-side of the frequency converter 7.
Then, a short-circuit current flows between phases in the secondary side of the wound-rotor induction machine 1 through the first short-circuit device 12, and the short-circuit current decays in accordance with the time constant of the secondary winding of the wound-rotor induction machine 1.
Next, the short-circuiting of the first short-circuit device 12 is terminated at the time t3 in consideration of the time when the failure in the electric power system 3 or the power transmission line 4 is eliminated. Since the first short-circuit device 12 is formed of a thyristor with high current tolerance, the first short-circuit device 12 cannot be turned off unless the short-circuit current becomes zero. The frequency converter 7 is restarted to apply a voltage in the direction opposite to the short-circuit current flowing through the first short-circuit device 12 so that the current flowing through the first short-circuit device 12 becomes zero.
However, with the conventional systems, when an inverse voltage is applied to cause the current flowing through the first short-circuit device 12 to be zero, there may be a case where the short-circuit current that was flowing through the first short-circuit device 12 flows into the frequency converter 7 at the time t4, the DC link voltage increases again, and the chopper 11 and the first short-circuit device 12 are reactivated. As a result, the temperature in the resistor 9 of the chopper 11, the element 10 and the thyristor of the first short-circuit device 12 may increase, and they may be broken.
There is a possible way to avoid this by increasing the rated values of the chopper 11 or the first short-circuit device 12. However, this increases the cost or requires oversizing of the chopper 11 or the first short-circuit device 12.
In addition, since the current flowing through the first short-circuit device 12 decays in accordance with the time constant of the secondary winding of the wound-rotor induction machine 1, the time required for the current to become zero will increase by the order of a few seconds as the reactance of the wound-rotor induction machine 1 will increase. Accordingly, the time required for restarting will increase.
Under the above circumstances, it is desired to provide an over-voltage prevention device and a current-rectifying circuit, which are capable of recovering from a failure in the electric power system within a short time and ensuring continuous operation with a simple structure.